The present invention relates to a zoom strobe device for a zoom camera.
A zoom strobe device is generally configured such that a light emitting unit including a light source and a reflector is moved relative to a light collecting lens in order to vary an illumination angle. A zoom strobe device built-in the zoom camera is configured to vary the illuminating angle so as to conform with a photographing angle. The light collecting lens generally has a positive power. In a conventional zoom strobe device, the light collecting lens is designed such that each refractive surface thereof has a positive power, or one has a positive power and the other has no power.
The conventional light collecting lens is configured such that a light emitting unit side surface is a convex surface or the Fresnel surface. It should be noted that, if the light emitting unit side surface of the light collecting lens is formed as a convex surface, in order to obtain a sufficient thickness at the periphery of the lens, the thickness of the entire lens increases, which is not preferable.
Further, an object side surface of the light collecting lens is formed as a planar surface, a convex surface or a convex lenticular surface.
The object side surface of the collection lens is formed as the planar surface in view of design of the strobe device and/or the camera.
Alternatively, in view of the design of the strobe device and/or the camera, the object side surface of the collection lens may also be formed as a convex surface. This structure has an advantage in that, by forming the object side surface as the convex surface, a guide number at a tele extremity side (i.e., when the zoom lens is located at a tele extremity side and therefore the illumination angle is relatively small) can easily be increased. In addition, if the object side surface of the collection lens is formed to be the convex lenticular surface, the inside of the strobe device may be prevented from being seen from the outside.
It should be noted that the object side surface of the light collecting lens cannot be formed as the Fresnel surface. If the object side surface is formed as the Fresnel surface, particles of dust or the like may enter the grooves or stepped portions thereof, and an illumination distribution performance may be deteriorated or the dust particles may burn. Therefore, the object side surface is generally formed as a smooth surface.
Recently, zooming cameras having a relatively large zooming ratio (i.e., a ratio of the maximum focal lengths to the minimum focal length) are preferred. For such cameras, an illumination zooming ratio (i.e., a ratio of the maximum illumination angle to the minimum illumination angle) of the zoom strobe device is required. However, the conventional zoom strobe device as described above has disadvantages as follows.
1. If the illumination zooming ratio is large, the guide number at the tele extremity side does not increase sufficiently;
2. If the strobe device is optimized to provide a sufficient guide number at the tele extremity side, the guide number may not increase sufficiently at a wide extremity side, or light amount in a peripheral area within the illumination angular range at the wide extremity decreases greatly.
In particular, in a recent compact camera having a relatively large zooming ratio, an F number is relatively large, especially at the tele extremity side, and therefore the guide number at the tele extremity side should be made sufficiently large. Accordingly, the guide number of the strobe device at the tele extremity side should be made large. In such a case, however, in order to obtain the sufficient illumination intensity distribution characteristic at the wide extremity side, manufacturing cost may increase since the structure of the camera may be enlarged, a large capacity condenser may be used and/or another type of light source may be employed to improve the light intensity distribution. It should be noted that the lowered intensity of light at the peripheral area is problematic on the wide side where the illumination angle is large, and is not problematic on the telephoto side where the illumination angle is small.
The present invention provides a zoom strobe device whose illumination intensity distribution characteristic at the wide side is improved.
According to one embodiment of the invention, there is provided a zoom strobe device, which is provided with a light emitting unit including a light source and a reflector and a light collecting lens that collects the light emitted by the light emitting unit, the light emitting unit and the light collecting lens being movable relative to each other in a direction of an optical axis of the light collecting lens. The light collecting lens is configured such that a light emitting unit side surface has a positive power, and a surface opposite to the light emitting unit side surface (i.e., the object side surface) has a negative power. The light collecting lens has a positive power as a whole.
By employing a surface having a negative power on the object side surface, the illumination intensity distribution characteristic at the wide side is improved and further, sufficient guide numbers can be obtained at wide and tele extremities.
In one embodiment, the light emitting unit side surface may be formed with a Fresnel lens surface.
According to embodiments, the light source may include a rod-like light source, and the light collecting lens has a shape of substantially a rectangular parallelepiped. In this case, a longer side of the light collecting lens may extend in a direction parallel to an extending direction of the rod-like light source.
In a particular case, the surface opposite to the light emitting unit side surface may be formed as a concave surface which is rotationally symmetrical with respect to the optical axis of the light emitting unit.
Alternatively, a surface opposite to the light emitting unit side surface is formed as a cylindrical concave surface. In this case, the generatrix of the cylindrical surface is on a plane perpendicular to an optical axis of the light emitting unit, and is parallel to or perpendicular to an extending direction of the rod-like light source.
In a particular case, the strobe device may be configured to satisfy condition:
(Haxe2x88x92Hb)/Ha less than 0.35, 
wherein, Ha represents a height of the collecting lens, on the rod-like light source side, in a direction perpendicular to the central axis of the rod-like light source and the optical axis of the light emitting unit, and
wherein Hb represents a length of an opening of the reflector on the collecting lens side, in the direction perpendicular to the central axis of the rod-like light source and the optical axis of the light emitting unit.